476 research outputs found
Van Der Waals Revisited
The van-der-Waals version of the second virial coefficient is not far from
being exact if the model parameters are appropriately chosen. It is shown how
the van-der-Waals resemblance originates from the interplay of thermal
averaging and superposition of scattering phase shift contributions. The
derivation of the two parameters from the quantum virial coefficient reveals a
fermion-boson symmetry in non-ideal quantum gases. Numerical details are worked
out for the Helium quantum gases.Comment: 11 pages, 4 figures, minor changes, accepted for publication in
Physica
Revisiting the chain magnetism in Sr14Cu24O41: Experimental and numerical results
We study the magnetism of the hole doped CuO2 spin chains in Sr14Cu24O41 by
measuring the Electron Spin Resonance (ESR) and the static magnetization M in
applied magnetic fields up to 14 T. In this compound, the dimerized ground
state and the charge order in the chains are well established. Our experimental
data suggest that at low temperatures the Curie-like increase of M as well as
the occurrence of the related ESR signal are due to a small amount of
paramagnetic centers which are not extrinsic defects but rather unpaired Cu
spins in the chain. These observations qualitatively confirm recent ab initio
calculations of the ground state properties of the CuO2 chains in Sr14Cu24O41.
Our complementary quantum statistical simulations yield that the temperature
and field dependence of the magnetization can be well described by an effective
Heisenberg model in which the ground state configuration is composed of spin
dimers, trimers, and monomers.Comment: revised versio
Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry
We investigate the magnetocaloric properties of certain antiferromagnetic
spin systems that have already been or very likely can be synthesized as
magnetic molecules. It turns out that the special geometric frustration which
is present in antiferromagnets that consist of corner-sharing triangles leads
to an enhanced magnetocaloric effect with high cooling rates in the vicinity of
the saturation field. These findings are compared with the behavior of a simple
unfrustrated spin ring as well as with the properties of the icosahedron. To
our surprise, also for the icosahedron large cooling rates can be achieved but
due to a different kind of geometric frustration.Comment: 5 pages, 8 figures, more information at
http://obelix.physik.uni-osnabrueck.de/~schnack
Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules
We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New
trends in magnetism and magnetic materials
Quantum numbers for relative ground states of antiferromagnetic Heisenberg spin rings
We suggest a general rule for the shift quantum numbers k of the relative
ground states of antiferromagnetic Heisenberg spin rings. This rule generalizes
well-known results of Marshall, Peierls, Lieb, Schultz, and Mattis for even
rings. Our rule is confirmed by numerical investigations and rigorous proofs
for special cases, including systems with a Haldane gap. Implications for the
total spin quantum number S of relative ground states are discussed as well as
generalizations to the XXZ model.Comment: 8 pages, 2 figures, submitted to Phys. Rev. B. More information at
http://www.physik.uni-osnabrueck.de/makrosysteme
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